1. Field of the Invention
This invention relates to the chemical maintenance of electroless copper plating solutions, and more particularly, to a method of and apparatus for eliminating bailout and thus the need for waste treatment in electroless copper purification by electrosynthesis/electrodialysis, and also for maintaining the stability of the electroless copper plating solution.
2. Description of the Prior Art
In the operation of an electroless copper plating bath, a number of bath constituents are consumed. These include copper (usually in the form of copper sulfate), sodium hydroxide, and formaldehyde. Replenishment of these constituents has been effected by adding at least two, and in some cases, three or more liquid concentrates to the bath. The addition of liquid concentrates causes the volume of the bath to grow giving rise to the need for bailout which must be treated and disposed of as hazardous waste. Such disposal not only is costly but gives rise, also, to environmental concerns.
It is known in the prior art, as disclosed in U.S. Pat. No. 4,289,597 issued on Sept. 15, 1981 to David W. Grenda and in U.S. application Ser. No. 691,095 filed by Emmanuel Korngold on Jan. 14, 1985, now U.S. Pat. No. 4,600,493, issued July 15, 1986, to utilize electrosynthesis/electrodialysis as a process by which formate and sulfate by-products produced as the result of the copper plating process are chemically removed from the plating bath and replaced with hydroxyl ions. This chemical action together with evaporation from the plating bath surface area, in addition to air sparging, is sufficient to eliminate the need for bailout over a range of plating production rates. Water evaporates from the plating bath due to its elevated, typically 120.degree. F., operating temperature. If the tank surface area is sufficiently great and replenishment rates (stabilizer, copper and formaldehyde) are within a certain range, no bail-out is necessary for an experimentally determined number of square feet of boards being plated. If more square feet of boards are plated than this experimentally determined number, or if a greater thickness of copper is plated, then bailout becomes necessary. There also is a problem with plating bath volume growth due to flushing of the connecting lines to the electrosynthesis/electrodialysis apparatus during cleaning. Little additional water volume can be added to the plating bath due to the inability to vary the evaporation rate from the plating bath surface.
Thus, there is a need and a demand for an improved method of and apparatus for eliminating the need for bailout with the electrosynthesis/electrodialysis electroless copper purification process at all plating loadings and plating thicknesses within the capacity of the process. The present invention was devised to fill the technological gap that has existed in the prior art in this respect.